Fix tempest flavors and DEFAULT_INSTANCE_TYPE

The flavor selection was broken if DEFAULT_INSTANCE_TYPE is defined
but not yet created, for example when it is created in local.sh.
This also has the side effect of setting flavor_ref_alt to the first
flavor where it was unset in the previous code.

Change-Id: I1fa48b3f90af45144c92298b6b07a4f7ee3b499f
Merge branch 'fix-tempest-flavors' into dt-test